Geomorphic Processes
1. Learning Objectives
After reading these notes, you will be able to:
2. Geomorphic Processes — Introduction
Internal forces
Land-building forces
Diastrophism + Volcanism
External forces
Land-wearing forces
Weathering, Erosion, Deposition
Wearing down of relief
through erosion
Filling up of basins
through deposition
Wearing down of relief
variations by erosion
General term for all
exogenic processes
⚙️ Endogenic Processes
🏔️ Diastrophism
All processes that move, elevate or build up portions of Earth’s crust. Includes: (i) Orogenic processes — mountain building through severe folding; (ii) Epeirogenic processes — uplift or warping of large crust parts; (iii) Earthquakes — local minor movements; (iv) Plate tectonics — horizontal movement of crustal plates. Causes PVT (Pressure, Volume, Temperature) changes → metamorphism of rocks.
🌋 Volcanism
Includes the movement of molten rock (magma) onto or toward Earth’s surface and formation of many intrusive and extrusive volcanic forms. (Dealt in detail in Unit II.)
☀️ Exogenic Processes — Key Points
- Derive energy from atmosphere determined ultimately by the Sun and from gradients created by tectonic factors.
- All exogenic geomorphic processes are covered under the general term Denudation (meaning ‘to strip off / uncover’).
- Denudation includes: Weathering, Mass wasting/movements, Erosion and Transportation.
- Two most important climatic elements controlling exogenic processes: Temperature and Precipitation.
- Intensity of exogenic processes also depends on type and structure of rocks — folds, faults, joints, hardness, permeability, etc.
- Effects of most exogenic processes are small and slow but severely affect rocks in the long run due to continued fatigue.
- Basic cause of weathering, mass movements and erosion: development of stresses in the body of earth materials.
3. Weathering
🧪 A. Chemical Weathering Processes
💧 Solution
Minerals that are soluble in water get dissolved and carried away. Common in limestone areas — forms karst topography.
💨 Carbonation
CO₂ dissolves in water to form carbonic acid which reacts with minerals (especially limestone/chalk) and dissolves them.
💦 Hydration
Water molecules are absorbed into the mineral structure, causing it to swell, crack and disintegrate. Affects feldspars strongly.
🔴 Oxidation
Minerals react with oxygen (usually from water/air) forming oxides. Iron-rich minerals form red/brown iron oxide — gives red colour to soils.
🔵 Reduction
Opposite of oxidation. Occurs in waterlogged or anaerobic conditions — minerals lose oxygen, producing blue-grey colours in soils.
🔨 B. Physical / Mechanical Weathering Processes
🌱 C. Biological Weathering
- Contribution to or removal of minerals from weathering environment and physical changes due to growth or movement of organisms.
- Burrowing and wedging by earthworms, termites, rodents — expose new surfaces to chemical attack and help penetration of moisture and air.
- Human beings — by disturbing vegetation, ploughing and cultivating soils, help mixing and creating new contacts between air, water and minerals.
- Decaying plant and animal matter produce humic, carbonic and other acids which enhance decay and solubility of some elements.
- Plant roots exert tremendous pressure on earth materials, mechanically breaking them apart.
⚡ Special Effect — Exfoliation
🌟 Significance of Weathering
- Breaks rocks into smaller fragments — prepares the way for formation of regolith, soils, erosion and mass movements.
- Biomes and biodiversity depend on forests; forests depend on depth of weathering mantles.
- Erosion cannot be significant if rocks are not weathered first.
- Weathering helps in enrichment and concentration of valuable ores — iron, manganese, aluminium, copper. This concentration is called enrichment.
- Weathering is an important process in formation of soils.
4. Mass Movements
⚠️ Conditions Favouring Mass Movements
- Weak unconsolidated materials and thinly bedded rocks
- Faults, steeply dipping beds, vertical cliffs or steep slopes
- Abundant precipitation and torrential rains
- Scarcity of vegetation
- Removal of support from below, increase in gradient, overloading, heavy rainfall, saturation, earthquakes, explosions, excessive seepage, indiscriminate removal of vegetation
📋 Types of Landslides (Rapid Mass Movements)
| Type | Description |
|---|---|
| Slump | Slipping of one or several units of rock debris with a backward rotation with respect to the slope |
| Debris Slide | Rapid rolling or sliding of earth debris without backward rotation of mass |
| Debris Fall | Nearly a free fall of earth debris from a vertical or overhanging face |
| Rock Slide | Sliding of individual rock masses down bedding, joint or fault surfaces. Very fast and destructive on steep slopes. |
| Rock Fall | Free falling of rock blocks over any steep slope keeping itself away from the slope. Affects only superficial layers. |
🌊 Three Forms of Mass Movement
❄️ Heave
Heaving up of soils due to frost growth and other causes.
🌊 Flow
Mass of material flows as a viscous body — includes mudflow, earthflow, solifluction.
↘️ Slide
Materials slide en masse along a distinct failure plane — includes landslides, rockslides, slumps.
5. Erosion and Deposition
🌊 Geomorphic Agents of Erosion
💨 Wind (Gaseous)
Controlled by climatic conditions. Major agent of erosion in arid and semi-arid regions.
💧 Running Water (Liquid)
Controlled by climatic conditions. Most powerful agent of erosion in humid regions.
🧊 Glacier (Solid)
Controlled by climatic conditions. Major erosional agent in polar and high-altitude regions.
🌊 Waves
Not controlled by climate. Work determined by location along coastal region (litho-hydro sphere interface).
🌑 Groundwater
Not controlled by climate. Work determined by lithological character of the region. Permeable + soluble rocks → Karst topography.
6. Soil Formation (Pedogenesis)
🌿 Process of Soil Formation
- Soil formation depends first on weathering — the weathering mantle (depth of weathered material) is the basic input.
- Weathered material first colonised by bacteria, mosses and lichens.
- Dead remains of organisms help in humus accumulation.
- Minor grasses, ferns grow → later bushes and trees through seeds brought by birds and wind.
- Plant roots penetrate down; burrowing animals bring up particles → material becomes porous and sponge-like.
- Finally a mature soil forms — complex mixture of mineral and organic products.
🔑 Five Soil-Forming Factors (PTCBT)
🪨 1. Parent Material (Passive)
Passive control factor. Can be in-situ weathered rock (residual soils) or transported deposits (transported soils). Soil formation depends on texture, structure, mineral and chemical composition of rock debris. Young soils show strong links with parent rock type.
🏔️ 2. Topography (Passive)
Passive control factor. Influences through exposure to sunlight and surface/sub-surface drainage. Steep slopes → thin soils. Flat upland areas → thick soils. Gentle slopes with slow erosion and good percolation → favourable for soil formation.
🌦️ 3. Climate (Active)
Active factor. Climatic elements: (i) Moisture — precipitation, evaporation, humidity; (ii) Temperature — seasonal and diurnal variations. Excess water → eluviation (downward transport) and illuviation (deposition below). Dry climates → hardpans. Wet equatorial → desilication. Tropical → kanker (CaCO₃ nodules).
🌿 4. Biological Activity (Active)
Vegetation and organisms add organic matter, moisture retention, nitrogen. Dead plants provide humus. Nitrogen fixation by bacteria (e.g., Rhizobium in leguminous plant roots). Earthworms, ants, termites, rodents rework soil up and down — important in soil formation.
⏰ 5. Time (Passive)
Passive control factor. Length of time soil-forming processes operate determines maturation and profile development. A soil becomes mature when all processes act for sufficiently long time developing a complete profile. Soils from recent alluvium or glacial till are young — poorly developed horizons.
🔑 Special Soil Terms
| Term | Meaning |
|---|---|
| Eluviation | Downward transportation of soil components through the soil by excess water |
| Illuviation | Deposition of eluviated materials in lower soil horizons |
| Desilication | Removal of silica from the soil — occurs in wet equatorial regions with very high rainfall |
| Hardpan | Crust formed in soil in dry climates when ground water is brought up by capillary action and evaporates leaving behind salts |
| Kanker | Calcium carbonate (CaCO₃) nodules formed in tropical climates and areas with intermediate precipitation |
| Humus | The finely divided organic matter of the soil provided by dead plants |
| Nitrogen Fixation | Process by which bacteria convert gaseous nitrogen from air into a chemical form usable by plants |
| Enrichment | Concentration of valuable minerals (Fe, Mn, Al, Cu) in residual soils after weathering removes other materials |
| Regolith | Layer of unconsolidated rock debris/weathered material on which soil develops |
Summary — Quick Revision
Endogenic forces = land-building (Diastrophism + Volcanism). Exogenic forces = land-wearing (Weathering, Mass movements, Erosion, Deposition). Together they shape Earth’s surface.
Denudation = general term for all exogenic processes. Includes weathering, mass wasting, erosion and transportation. Driven ultimately by solar energy and tectonic gradients.
Orogeny = mountain-building (folding of narrow belts). Epeirogeny = continental-building (uplift/warping of large crust portions). Both are part of diastrophism.
Weathering = in-situ process. 3 types: Chemical (solution, carbonation, hydration, oxidation, reduction), Physical (thermal expansion, pressure release), Biological (organisms, plant roots).
Exfoliation = flaking off of curved sheets from rocks — smooth rounded surface results. It is a result, not a process. Exfoliation domes = unloading; Tors = thermal expansion.
Significance of weathering: prepares regolith/soil, aids erosion, supports biomes and biodiversity, and causes enrichment of mineral ores (Fe, Mn, Al, Cu).
Mass movements = debris moves under direct gravity. No geomorphic agent involved. 3 forms: Heave, Flow, Slide. Landslide types: Slump, Debris slide, Debris fall, Rockslide, Rock fall.
Erosion agents: Wind (gaseous), Running water (liquid), Glaciers (solid) — climate-controlled. Waves and Groundwater — not climate-controlled. Deposition = consequence of erosion (loss of energy).
Soil formation (pedogenesis) depends first on weathering. 5 factors: Parent material, Topography, Climate, Biological activity, Time. Active: Climate + Bio. Passive: Parent material, Topography, Time.
Key soil terms: Eluviation (downward transport), Illuviation (deposition below), Desilication (silica removal), Hardpan (salt crust in dry climate), Kanker (CaCO₃ nodules in tropical climate).
Important Terms to Remember
- Geomorphic Processes: Endogenic and exogenic forces causing physical stresses and chemical actions on earth materials, bringing about changes in the configuration of Earth’s surface.
- Geomorphic Agent: A mobile medium (running water, glaciers, wind, waves, currents) that removes, transports and deposits earth materials.
- Exogenic Forces: External forces originating from the atmosphere; mainly land-wearing forces. Ultimately driven by solar energy. Include weathering, mass movements, erosion and deposition.
- Endogenic Forces: Internal forces originating from within the Earth; mainly land-building forces. Driven by radioactivity, tidal friction and primordial heat. Include diastrophism and volcanism.
- Degradation: Wearing down of relief/elevations through erosion by exogenic processes.
- Aggradation: Filling up of basins/depressions through deposition by exogenic processes.
- Gradation: The phenomenon of wearing down of relief variations of the surface of the earth through erosion.
- Denudation: General term for all exogenic geomorphic processes — includes weathering, mass wasting, erosion and transportation. Means ‘to strip off or uncover’.
- Stress: Force applied per unit area on earth materials. Shear stresses separate and break rocks.
- Diastrophism: All processes that move, elevate or build up portions of Earth’s crust — includes orogeny, epeirogeny, earthquakes and plate tectonics.
- Orogeny: Mountain-building process involving severe folding of the crust in long and narrow belts.
- Epeirogeny: Continental-building process involving uplift or warping of large parts of the Earth’s crust.
- Volcanism: Movement of molten rock (magma) onto or toward Earth’s surface and formation of intrusive and extrusive volcanic forms.
- Weathering: Mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate. An in-situ (on-site) process — very little or no movement of material.
- Chemical Weathering: Decomposition of rocks through chemical reactions — processes include solution, carbonation, hydration, oxidation and reduction.
- Physical / Mechanical Weathering: Disintegration of rocks by applied forces — gravitational forces, expansion forces (temperature changes, crystal growth), water pressure. Mostly caused by thermal expansion and pressure release.
- Biological Weathering: Weathering through contribution/removal of minerals by organisms and physical changes due to growth or movement of organisms. Earthworms, termites, plant roots, humans all contribute.
- Exfoliation: Flaking off of more or less curved sheets from over rocks resulting in smooth and rounded surfaces. It is a result, not a process. Exfoliation domes (unloading); Tors (thermal expansion).
- Regolith: The layer of unconsolidated weathered rock debris on which soil eventually develops.
- Enrichment: Concentration of valuable mineral ores (Fe, Mn, Al, Cu) in residual soils after weathering removes other soluble materials through chemical/physical leaching.
- Mass Movements: Transfer of mass of rock debris down slopes under direct influence of gravity. No geomorphic agent participates. Not classified as erosion. Three forms: heave, flow and slide.
- Slump: Slipping of one or several units of rock debris with a backward rotation with respect to the slope. A type of landslide.
- Debris Slide: Rapid rolling or sliding of earth debris without backward rotation of mass.
- Rockslide: Sliding of individual rock masses down bedding, joint or fault surfaces. Very fast and destructive on steep slopes.
- Rock Fall: Free falling of rock blocks over steep slopes. Occurs from superficial layers — distinguishes it from rockslide which goes deeper.
- Erosion: Acquisition and transportation of rock debris by geomorphic agents. Erosion degrades the landscape (wears it down). Weathering aids but is not a pre-condition for erosion.
- Deposition: Consequence of erosion. Erosional agents lose velocity and energy on gentler slopes; carried material settles. Coarser materials settle first, finer ones later. Fills up depressions.
- Pedology: Soil science. A pedologist is a soil scientist.
- Pedogenesis: The process of soil formation.
- Humus: Finely divided organic matter in soil, provided by decaying dead plants.
- Eluviation: Downward transportation of soil components through the soil by excess water.
- Illuviation: Deposition of eluviated materials in lower horizons of the soil.
- Desilication: Removal of silica from the soil — occurs in wet equatorial areas with very high rainfall.
- Hardpan: A crust of salts formed in soil in dry climates when capillary action brings groundwater to the surface, which then evaporates leaving salts behind.
- Kanker: Calcium carbonate (CaCO₃) nodules formed in tropical climates and areas with intermediate precipitation.
- Nitrogen Fixation: Process by which bacteria (e.g., Rhizobium in root nodules of leguminous plants) convert atmospheric nitrogen into chemical forms usable by plants.
- Residual Soils: Soils formed from in-situ (on-site) weathered rock debris of the underlying bedrock.
- Transported Soils: Soils formed from deposits transported and laid down by water, wind, glaciers, etc., away from the parent rock.